The present invention relates to a metal loss measurement system for the detection of corrosion and for measuring the rate of metal mass loss. The invention may be applied generally to the detection of metal-loss by corrosion and/or erosion species in single or multiphase fluids. In particular, the present invention relates to the on-stream detection of metal-loss corrosion and/or erosion during an industrial production process. The actual service environment may be aqueous, hydrocarbon, chemical or a combination.
Corrosive species involved in the production and processing of crude oil and hydrocarbons may cause metal-loss corrosion of production, transfer, storage, and processing equipment. Erosive species typically involve fluid and/or solids turbulence causing metal loss from mechanical actions rather than chemical. For example, these corrosive/erosive species may be hydrocarbon, hydrocarbon containing materials, or aqueous, or combinations thereof. Moreover, streams may be single or multi-phase (solids, liquids, gases).
High performance, relatively low-cost corrosion (erosion) sensing technology as in the instant invention would enable, for example, optimization utilization of corrosive crudes and corrosion inhibitor additions, and reductions in unplanned capacity loss, turnaround time, and inspection costs due to corrosion-induced equipment failures. Additional value is achievable with the application of the instant invention to corrosion monitoring of transfer, process, and storage equipment used for crude oil, fractions and derived products, and chemicals and other industries concerned with corrosion and erosion. Further value is achievable with the application to monitoring metal-loss corrosion in equipment used for the extraction of crude oil from subsurface and subsea deposits. In these and other services, a by-product of the corrosion is scale or other depositions that are adherent to the containment surface. A feature of the instant invention is that the metal loss measurement is not compromised by these non-metallic depositions.
Current corrosion sensing technologies, for example electrical resistance probes, fall far short of the performance level required to achieve the economic incentives described above. Their shortcomings are that: One limitation relates to sensitivity versus useful sensor life. Increasing sensitivity of the conventional electrical resistance probe is achieved by decreasing the thickness of the sensing element. However, the decrease in thickness results in a reduced life of the probe. Once corrosion results in a breach of the element, the probe will no longer function and must be replaced. In an operating process unit, on-stream probe replacement poses various safety and hazard issues. Another limitation of the electrical resistance probes relates to their inherent signal variability. The signal variability caused by thermal changes and other factors that affect electrical resistance necessitate long data collection periods (often a week or longer) to establish a reliable trend. While conventional electrical resistance probes are based on understood theoretical principals, these probes often provide low reliability and poor sensitivity to corrosion rates due to limitations in their design and manufacture. The typical output is often difficult for estimating a quantitative corrosion rate.
It is well established in the literature (references 3 and 4) that the lightly damped harmonic oscillator with single degree of freedom can be mathematically expressed by a second order differential equation. If the forcing function is sinusoidal, the resonance frequency and quality factor, Q, can be represented by:fo=(1/2π)√(k/m)Q=(1/c)*√(k*m)Where    m=system mass    k=system stiffness    c=velocity dependent damping    fo=resonance frequency    Q=Quality factor (a measure of the system damping and energy dissipation)There is an implicit assumption that the damping is light so that the mechanical resonance can be observed.
U.S. Pat. No. 6,928,877 and US application 2006/0037399 both employ resonators and teach a relationship between the resonance frequency and mass change. The relationship taught by the prior art is consistent with the well-known solution described above for a single degree of freedom lightly damped mechanical oscillator: a mass decrease will result in a frequency increase and a mass increase will result in a frequency decrease. The instant invention teaches away from the prior art by discovering and utilizing that mass decrease from corrosion/erosion can also result in a resonance frequency decrease. Clearly this finding is not obvious in light of the teachings of the prior art. However, this finding is actually consistent with the governing equations previously listed. The instant invention has utilized that the stiffness of the resonator device is also governed by the system mass. Moreover, that relationship between system mass and stiffness is location dependent: the amount of change to the system stiffness is dependent upon where the mass is lost (or gained). The instant invention teaches that by selecting the proper location on the vibrating element, it is possible that the change in the stiffness to mass ratio of equation (1) can be stiffness dominated even though mass is being lost. For that case, a loss of mass will result in a frequency decrease, teaching away from the prior art.
U.S. Pat. No. 6,928,877 also teaches to make the mass additions or losses at the tip of the resonator. The instant invention teaches away from making the tip the mass change location. In fact, the instant invention provides details for minimizing any mass change at the resonator tip. For the instant invention, the resonator's mass change location is designed to be close to tine the attachment point. At this location, mass loss has a sufficient impact on system stiffness as to cause a resonance frequency decrease. Essentially, mass change at this location on the tuning fork resonator is controlling the system resonance frequency because of the corresponding change to stiffness. Moreover, mass addition from corrosion scale or fouling near the base has minimal impact on the resonance frequency because scale and fouling do not significantly contribute to system stiffness. This observation is understood because the Young's modulus (a measure of a material's stiffness) of scale or fouling is several orders of magnitude lower than steel. However, if scale or fouling deposition were to occur near the resonator tip, the added mass would result in a frequency decrease. Any change to the system stiffness is overshadowed by the mass change.
US application 2006/0037399 also teaches away from employing stiffness changes to the resonator inherent mass by the use of installing corrodible material in pockets. That application describes the use of pockets to facilitate the periodic replacement of corrodible elements installed in the pockets. The ability to employ the device stiffness of the instant invention is dependent upon the corrodible element being an integral element of the resonator (e.g. attached securely via welding). Corrodible elements attached by means of pockets in US application 2006/0037399 would not meet the criteria enabling a stiffness change to the resonator. Attachment via pockets can only enable a mass change.
U.S. Pat. No. 6,928,877 does not consider the situation of simultaneous metal mass loss and fouling deposition. US application 2006/0037399 teaches that mass loss will increase the resonance frequency and that fouling deposition will decrease the resonance Q. Therefore, 2006/0037399 teaches that the fouling condition in the presence of mass loss (corrosion or erosion) can be recognized by the Q measurement. The application teaches that a neural network or artificial intelligence can be used to infer corrosion and fouling conditions from the resonance parameters. The instant invention teaches away from these empirical and unreliable approaches by designing a probe where the resonance frequency is primarily dependent upon corrosion (steel) mass loss and is substantially insensitive to deposition from fouling or corrosion products.
The focus of U.S. Pat. No. 6,928,877 and 2006/0037399 is to provide a quantitative estimate of mass loss or deposition. Essentially, both provide an alarming function. The instant invention has sufficient precision, stability, and longevity as to provide a mass loss rate quantity.